The present invention relates broadly to hearing protectors of the earplug type and is more specifically concerned with earplugs composed of plasticized thermoplastic foam materials.
Plasticized thermoplastic foam earplugs are known from such literature as British Pat. No. 733,542, to Gustav Hultgren, filed Oct. 16, 1953 and U.S. Pat. No. Re. 29,487, to Gardner, Jr., issued Dec. 16, 1977. Hultgren discloses earplugs formed of a soft spongy material which preferably consists of a softened vinyl chloride polymer or copolymer. The noise damping effects of the earplugs are disclosed to be controllable through control of the density and pore size of the spongy material of construction. Gardner, Jr. discloses earplugs composed of a resilient plasticized polymeric foam, such as polyvinyl chloride or polyurethane. The polymeric foam material of construction contains a sufficiently high concentration of organic plasticizer as to provide said material with certain rate of recovery and equilibrium pressure properties and, by virtue of this combination of properties, the earplugs of Gardner, Jr., upon compression thereof, are readily insertable into human ear canals whereupon they expand to provide comfortable and uniform sealing of the canals and act as highly competent sound barriers.
For use in certain environments it is often desirable to provide earplug type hearing protectors with suitable means to prevent their loss should they work loose or otherwise be dislodged. For instance, this is an important feature to provide for workers in the food processing industry wherein all due care must be exercised in preventing foreign matter from entering the foodstuff products. Likewise, those who work at substantial elevations or heights, such as iron and steel workers, are also desirably provided with hearing protector wares which are not susceptible of accidental loss.
It is known to provide earplug constructions comprising a pair of earplugs which are tethered together in spaced relation by means of a cord element which may be secured to the user's person or clothing. However, earplug constructions of this type have, heretofore, been essentially limited to those earplugs composed of unfoamed polymeric or elastomeric materials. Unfortunately, these unfoamed earplugs generally do not confer as uniformly competent hearing protection as do plasticized polymeric foam earplugs such as taught in the hereinbefore-mentioned Gardner, Jr. patent. It has been proposed to thermally weld a flexible thermoplastic cord to earplugs composed of plasticized thermoplastic foam materials. However, this has not heretofore been successfully accomplished on a practical assembly line basis to the knowledge of the present applicant. Indeed, the art has generally recognized that ultrasonic sealing or welding of plasticized polyvinylchloride materials cannot normally be achieved. For instance, at page 1345 of Volume 3 of The Encyclopedia of PVC, edited by Leonard I. Nass, Marcel Dekker, Inc., New York, 1977 it is stated: "Ultrasonic sealing cannot be used with flexible or plasticized PVC. The high-frequency sonic vibrations are not effective in generating the heat of fusion of soft substances." It also appears that one aspect of the problem resides in the tendency of thermoplastic polymeric foam materials to rapidly collapse upon application of welding heat thereto, thereby to result in substantial distortion of the earplug and to result in an excessively weak weldment or union between the earplug and the thermoplastic cord element. In accordance with the present invention, however, this problem has been solved.